Added 'AttrToSymbol' option; other changes

Hi, Maik,
I only recently found XmlSimple, and so far I love it! It only needed a few _tiny_ changes, hence this pull request.

To go with the KeyToSymbol option, I added an AttrToSymbol option. I also renamed xmlsimple.rb to xml-simple.rb, since it was confusing to me that the require'd file didn't match the gem name. I also updated _some_ of the documentation (I didn't do anything to the RDoc, since I didn't change the API).

Let me know if you want anything changed before you pull (assuming you want to at all). BTW, sorry about the "master branch" thing. This is my first pull request, and I'm not very experienced with git as a whole (I'm not even sure what a "topic branch" is, much less how to make one :P ).

-Amos